Effect of altitude on vertical temperature distribution and longitudinal smoke layer thickness in long tunnel fires

Based on large eddy simulation, a series of long tunnel fire experiments with different heat release rates (HRRs) and altitudes were carried out. The vertical temperature and thickness of fire smoke are studied. The simulation results show that the higher the altitude, the lower the flame temperature rise, while the change of smoke plume temperature rise is opposite. The movement of smoke in the tunnel can be divided into four regions, and the smoke layer thickness in the longitudinal direction of the tunnel corresponds to the latter three regions. The thickness in Region II increases along the longitudinal direction, the thickness in Region III is a constant value, and the thickness in Region IV increases along the longitudinal direction. Besides, the change of altitude only has an effect on the smoke layer thickness in Region IV. Then, by considering the altitude, HRR, and smoke layer thickness, and using dimensional analysis, an empirical formula for predicting the smoke layer thickness under the influence of altitude in Region IV was established.     

Experimental study of the combustion characteristics of methanol‐gasoline blends pool fires in a full‐scale tunnel

The combustion characteristics of methanol‐gasoline blends pool fires were studied in a series of full‐scale tunnel experiments conducted with different methanol and gasoline blends. The parameters were measured including the mass loss rate, the pool surface temperature, the fire plume centerline temperature, the ceiling temperature, the smoke layer temperature profile, the flame height, and the smoke layer interface height. The gasoline components were analyzed by GC‐MS. The effects of azeotropism on the combustion characteristics of the different blends were discussed. On the basis of the results of the fire plume centerline temperature, the ceiling temperature, and the flame height, it shows that the tunnel fire regime gradually switches from fuel controlled to ventilation controlled with increasing gasoline fractions in the blends. The fire plume can be divided into 3 regions by the fire plume centerline temperature for the different blends. The N‐percentage rule to determine the smoke layer interface height is found to be applicable for tunnel fires with different blends for N = 26.     

Study of smoke movement characteristics in tunnel fires in high-altitude areas

Understanding smoke temperature distributions and transport characteristics is of great importance to control and exhaust thermal-driven smoke. However, previous studies have focused on this problem in plain areas, whereas ambient pressure decreases as elevation increases. This study investigates the influence of ambient pressure on the hot gas temperature distribution and movement characteristics in a tunnel fire. A series of numerical simulations are carried out in a vehicle tunnel with various heat release rates (HRRs) and ambient pressures. The results show that the maximum temperature and longitudinal temperature distribution under the tunnel ceiling increase with decreasing ambient pressure due to less heat loss caused by lower air density. In addition, the vertical temperatures of the smoke are slightly higher under lower ambient pressure, and this phenomenon makes the smoke spread slightly faster while the smoke layer thickness remains nearly the same under different ambient pressures. The results can provide a reference for tunnel lining design and ventilation arrangements in high-altitude areas.     

Experimental study on heat release rate measurement in tunnel fires

Knowledge about the heat release rate (HRR) is essential for studying tunnel fires. The standard method in ISO 9705 is widely applied to calculate the HRR of combustion by measuring the consumption of oxygen in a fire. However, the studies of HRR measurement in full‐scale tunnel fires are rare because of the complication and costs of large experiments. This paper presents a system based on the principle of oxygen consumption calorimetry for the measurement of HRR and total heat release (THR) of full‐scale fires in tunnels. A total of 22 fire experiments are performed in a large‐scale ventilated testing metro tunnel with dimension of 100.0 m × 5.5 m × 5.5 m to validate the reliability and effectiveness of this system. Firstly, four oil spray fire tests are conducted with nozzle flow of 106 L/h at (1 ± 0.1) MW HRR to calibrate the instrumentation. Then, 18 full‐scale fire tests using square diesel pools at five sizes (0.5, 1.0, 2.5, and 5.0 m²) and wood cribs as fire sources are carried out for the measurement of HRR and THR. Results provided by the comparison between the measured HRR and THR values of the fire tests and the theoretically calculated ones show that our system works effectively in the HRR measurement of full‐scale fires in tunnels.     

Experimental study on the effect of lateral concentrated smoke extraction on smoke stratification in the longitudinal ventilated tunnel

Smoke is the main cause of death in tunnel fires. It is one of the important measures to maintain smoke stratification in the early stage of tunnel fire. This article focused on experimentally studying the combined effect of lateral concentrated smoke extraction and longitudinal ventilation on the smoke stratification, which never be revealed before. The velocity of the smoke layer and air layer, vertical temperature distribution, and the flow patterns of the smoke were measured. It was found that the longitudinal ventilation and lateral concentrated smoke extraction would affect the flow of the smoke and change the shear velocity between the smoke layer and air layer, then, the patterns of the smoke layer will be affected. And the flow patterns with Froude (Fr) number can be classified into three categories: (a)Fr < 0.6 , with stable smoke stratification; (b) 0.6 < Fr < 0.85 , with a stable smoke stratification but the blurring interface; and (c) Fr > 0.85 , the smoke layer is completely unstable. The result can provide a reference for ventilation design of immersed tube tunnels.     

Mechanism of smoke suppression by melamine in rigid polyurethane foam

The smoke suppression of rigid polyurethane foam (RPUF) modified by melamine was investigated based on three sections: the condensed phase, the carbon layer, and the gas phase. In the condensed phase, the results of thermogravimetry, X‐ray photoelectron spectroscopy (XPS) N1S spectrum, and Fourier transform infrared spectroscopy indicated that melamine could suppress the degradation of RPUF by reacting with the aromatic hydrocarbons. It also reduced the smoke generation because the volatilizable aromatic hydrocarbons were the principal smoke precursors in a fire. In the carbon layer, the decrease from 38.50% to 24.76% of the inner layer oxygen content identified by XPS full‐spectrum and C1S spectrum indicated that melamine could prevent oxygen from transferring into the inner foam by the formation of an enhanced surface carbon layer, and the enhanced carbon layer could also block the release of smoke precursors. In the gas phase, the content of total aromatic hydrocarbons declined to 59.12% according to pyrolysis gaseous chromatography mass spectroscopy and indicated that melamine could reduce the smoke precursors. The results of smoke density chamber and cone calorimeter tests revealed that the addition of the melamine could decrease the smoke density of burning RPUF. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of longitudinal fire location on smoke characteristics under the tunnel ceiling

The critical ventilation velocity is almost the most well‐investigated fire phenomenon in the tunnel fire research field whereas previous studies have always investigated it when the fire source is distant from the downstream tunnel exit. Fortunately, a recent study provided a set of data on the critical ventilation velocity for tunnel fires occurring near tunnel exits by small‐scaled experiments, nevertheless, with a lack of further analysis. To demonstrate the relationship of the critical ventilation velocity and the distance between the fire and tunnel exit more explicitly and detailedly, a quantitative and graphical study was carried out and a correlation was presented in this paper. Inspired by this, a set of small‐scaled experiments were carried out to investigate the influence of different longitudinal fire locations on maximum smoke temperature under the tunnel ceiling. Results show that unlike the critical ventilation velocity, the maximum smoke temperature was not obviously affected by longitudinal fire location. Copyright © 2014 John Wiley & Sons, Ltd.     

CFD modeling approach of smoke toxicity and opacity for flaming and non‐flaming combustion processes

Current engineer's methods of fire safety design include various approaches to calculate the fire propagation and smoke spread in buildings by means of computational fluid dynamics (CFD). Because of the increased computational capacity, CFD is commonly used for prediction of time‐dependent safety parameters such as critical temperature, smoke layer height, rescue times, distributions of chemical products, and smoke toxicity and visibility. The analysis of smoke components with CFD is particularly complex, because the composition of the fire gases and also the smoke quantities depends on material properties and also on ambient and burning conditions. Oxygen concentrations and the temperature distribution in the compartment affect smoke production and smoke gas toxicity qualitatively and quantitatively. For safety designs, it can be necessary to take these influences into account. Current smoke models in CFD often use a constant smoke yield that does not vary with different fire conditions. If smoke gas toxicity is considered, a simple approach with the focus on carbon monoxide is often used. On the basis of a large set of experimental data, a numerical smoke model has been developed. The developed numerical smoke model includes optical properties, production, and toxic potential of smoke under different conditions. For the setup of the numerical model, experimental data were used for calculation of chemical components and evaluation of smoke toxicity under different combustion conditions. Therefore, averaged reaction equations were developed from experimental measurements and implemented in ANSYS CFX 14.0. Copyright © 2015 John Wiley & Sons, Ltd.     

Study on the heat exhaust coefficient and smoke flow characteristics under lateral smoke exhaust in tunnel fires

The heat exhaust coefficient and smoke flow characteristics under lateral smoke exhaust in tunnel fires were studied in this paper. Through the dimensional analysis, the dimensionless relationship between the heat exhaust coefficient, heat release rate, exhaust vent size, and exhaust velocity was obtained. In addition, this paper also studied the effect of the lateral exhaust vent on the smoke flow field. Results showed that the lateral smoke exhaust caused strong air entrainment on the downstream of the exhaust vent and boundary layer separation on the upstream of the exhaust vent. As the exhaust velocity increased, the degree of air entrainment gradually increased, and the smoke layer near the exhaust vent gradually became thinning and plug‐holing phenomenon occurred; meanwhile, the boundary layer separation would be suppressed or disappear, but the increase of the heat release rate would enhance the boundary layer separation. As the exhaust vent got narrower, the air entrainment downstream of the exhaust vent was reduced, and the boundary layer separation also got weaker.     

New ecological polyester resins with reduced flammability and smoke evolution capacity

Smoke evolution in a smoke chamber 750 × 750 × 1000 ± 5 mm (Polish Standard PN‐91/K‐02501 equivalent to UIC 561‐OR 1991) was studied in, and the oxygen index flammability test (Polish Standard PN‐76/C‐89020) was carried out for, glass‐reinforced polyester (GRP) laminates obtained with unsaturated polyester (UP) resins containing chlorine and bromine in the chain. In these studies, the effect on these properties of such additives as ZnSnO₃ (ZS), ZnSn(OH)₆ (ZHS), Al(OH)₃, or Mg(OH)₂ and Sb₂O₃ in up to 30 mass % was determined. The most efficient ignition and smoke‐evolution retarder from among the investigated compounds were ZS and ZHS, and an essential reduction in smoke evolution was also observed with Sb₂O₃. GRP laminates with these additives meet the fire safety recommendations concerning smoke evolution for materials used in transportation and in the building industry. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 379–382, 1999     

Preparation of carbon/carbon‐ultra high temperature ceramics composites with ultra high temperature ceramics coating

In order to increase the oxidation resistance of carbon/carbon (C/C) composites at long‐term high temperature, C/C‐Ultra High Temperature Ceramics composites (UHTCs) with a dual‐layer UHTCs oxidation coating was successfully designed and fabricated. The microstructure and ablation resistance were investigated and discussed. After ablation in arc‐heated wind tunnel with temperature being 2200°C for 1000s, the mass ablation rate and linear ablation rate were ?1.9 × 10^?2 mg/cm²s and 2.9 × 10^?5 mm/s, respectively. The formation of thermodynamically compatible oxide scale including ZrO₂ skeleton and SiO₂ or Zr–Si–O glass on the surface were mainly contributed to the excellent ablation resistance of the composite.     

Inkjet Printing of Microporous Silica Gas Separation Membranes

Inkjet printing was applied to manufacture silica‐based gas separation membranes, which were coated on a pore‐graduated alumina substrate with a mesoporous γ‐alumina interlayer. A silica sol diluted by 1‐propanol was used to print the membrane layer followed by thermal treatment in a rapid thermal processing furnace. The membrane thickness was varied between 30 and 110 nm by conducting one, two, and three coating steps. In the latter case, H₂ permeance in the range of 2.0 × 10^?8–3.3 × 10^?8 mol/s·m²·Pa combined with H₂/CO₂ selectivities in the range of 15–25 were achieved, proving the concept that inorganic gas separation membranes can be successfully processed by inkjet printing.     

Effects of passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires

This paper investigates the effects of passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires. A series of numerical simulations were conducted in a 1/5 small-scale tunnel with the different heat release rates (50-100 kW), longitudinal ventilation velocities (0.5-1 m/s), passenger blockage lengths (2-6 m), and ratios (0.17-0.267). The typical smoke flow properties in different tunnel fire scenarios are analyzed, and the results show that under the same heat release rate and longitudinal ventilation velocity, the smoke back-layering length, maximum smoke temperature, and downstream smoke layer height decrease with increasing passenger blockage length or ratio. The Li correlations can well predict the smoke back-layering length and maximum smoke temperature in tunnel fire scenarios without the passenger blockage. When the passenger blockage exists, the modified local ventilation velocity that takes the blockage length and ratio into account has been proposed to correct the Li correlations. The smoke back-layering length and maximum smoke temperature with the different blockage lengths and ratios can be predicted by the modified correlations, which are shown to well reproduce the simulation results.     

Evaluating methods for preventing smoke spread through ventilation systems using fire dynamics simulator

Fires in enclosures equipped with mechanical ventilation remain one of the key issues for fire safety assessment in multifamily homes and industries. Therefore, a wide variation of methods for preventing smoke spread through the ventilation system exist and are applied, in performance‐based designs. Through the use of the heating, ventilation and air conditioning (HVAC) model in the fire dynamics simulator, several different common and less common methods for preventing smoke spread in the ventilation system were tested. The effects on smoke spread with changing building leakage and fire growth rates were also investigated. The results were evaluated by determining the total soot spread from the fire room to other compartments connected to the ventilation system, as well as soot/thermal load on the fans and system in general. The maximum and average heat release rate was also of interest and hence compared between systems. It was found that, while many methods perform similar, a few proven methods, such as fire and smoke dampers, performed very well with very little smoke spread to the rest of the system. The study should be considered as an introduction to implementing a similar methodology in specific cases because different ventilations systems will present very different challenges and weaknesses. Copyright © 2016 John Wiley & Sons, Ltd.     

Indicative fire tests to investigate heat insulation scenario of wired glasses sprayed with intumescent coatings

This study used three fire exposure samples with areas of 1 m × 1 m to conduct standard furnace fire tests. Through radiometer and theoretical analysis, the heat insulation of wired glass sprayed with intumescent coatings was measured. Test results indicate that after using an intumescent coating with a thickness of 0.3 mm, wired glass surface will generate a heat insulation layer that will dramatically reduce radiant energy levels due to thermal expansion. Results from this study suggest the use for a grid shape of intumescent coating because of the ease of application to wired glass and the fact that wired glass with an intumescent coating has proven to be the most feasible heat insulation material that can be practically applied in designated fireproof compartments of buildings.     

Structural and electrical properties of H-terminated diamond field-effect transistor

A dielectric barrier separating hydrogen induced p-type channel and Al gate metal contact of diamond FET has been investigated. The separation barrier is necessary to prevent tunneling current between the H-induced channel and the gate contact. In this investigation, CV measurements, fitting of forward IV characteristics, TEM and SIMS profiles have been used to obtain a more detailed picture of this barrier layer. While the composition of this layer is not clear, TEM and SIMS measurements indicate that this layer may be connected to a diamond phase or aluminium oxide. Using material properties of these materials, thickness of the separation layer extracted from the CV measurements was between 5–10 nm and the channel sheet change density was above 1 × 10¹³ cm^? 2. This thickness is in good agreement with the TEM observations. Frequency dependent CV measurements showed almost no frequency dependence, and no UV light dependence has been observed. Temperature dependent CV measurements showed a decrease of the dielectric constant at 100 °C. Fitting of the forward tunnelling current indicated a thickness of the barrier layer of about 5 nm with a barrier height of 2.4 eV.     

High-performance indium-free flexible transparent ATO/Au/ATO tri-layer films by magnetron sputtering

Due to the scarcity of indium (In) in the earth and its potential harm to individuals, the development of In-free transparent conductive film is considered crucial. In this work, In-free SnO₂:Sb/Au/SnO₂:Sb (ATO/Au/ATO, SAS) tri-layer films with high transparency and conductivity were successfully prepared on polycarbonate (PC) substrates by RF and DC magnetron sputtering at room temperature. The influence of the Au layer thickness on microstructure, electrical and optical performances was systematically studied after fixing the ATO thickness to 50 nm. It was indicated by X-ray diffraction patterns that ATO is amorphous and Au is oriented along (111). The trend of increasing and then decreasing light transmission with Au layer thickness was observed in both experimental and simulation results. The improved figure of merit (FoM, 1.89 × 10^?2 Ω^?1) was achieved in SAS tri-layer film, the resistivity and average transmittance of which was lowered to 7.50 × 10^?5 Ω cm and 81.4%, respectively, when Au layer thickness is 11 nm. Moreover, the mechanism of the variation of optical and electrical properties at different Au layer thickness was proposed. Particularly, the SAS tri-layer films also exhibit superior flexibility, durability and adhesion. These results demonstrate SAS tri-layer films are promising alternative to ITO in flexible electronics applications.     

Influence of magnetic field upon the electric capacity of a flat capacitor having magnetorheological elastomer as a dielectric

The flat capacitor is built of two non-magnetic plates (with dimensions 0.065 m × 0.050 m) between which there is a layer of magnetorheological elastomer (MRE). The thickness of the layer is 0.0015 m ± 10%. MRE is based on silicone rubber and iron particles. The iron particles diameter ranges between 0.12 μm and 0.75 μm. The electric capacity, in absence of the magnetic field, is 377 ± 1 pF. In cross magnetic field with strengths up to 94 kA/m, the flat capacitor's capacity increases by up to 200%. For well chosen values of the intensity of the magnetic field, the capacity of the flat capacitor with MRE changes with time. The experimental results obtained in this manner are discussed.     

Insightful investigation of smoke suppression behavior and mechanism of polystyrene with ferrocene: An important role of intermediate smoke

Aiming to investigate the smoke suppression mechanism of ferrocene in model polystyrene (PS) during combustion, we exploited the vapor‐phase and condensed‐phase behaviors. Cone calorimeter testing result showed that 3 wt% ferrocene imparted PS with 56.9% reduction in total smoke production. The analysis of the char after cone calorimeter testing demonstrated that the condensed‐phase smoke suppression mechanism was weak owing to the absence of charring behavior. The vapor‐phase mechanism was focused on the analysis of the small‐molecule smoke precursors and initially formed smoke intermediates. Transmission electron microscopy of initially formed smoke intermediates of PS/3Ferrocene revealed that enormous γ‐Fe₂O₃ nanoparticles from ferrocene participated in the initial formation of smoke intermediates, which subsequently underwent notable thermal oxidation degradation with decreased smoke residue. Thermogravimetric analysis coupled with Fourier transform infrared spectroscopy results manifested that the small‐molecule smoke precursors remained almost unchanged with addition of ferrocene. Conclusively, the smoke suppression mechanism with ferrocene predominantly originated from the intensive thermal oxidation of smoke intermediates, which opened a viable route for excellent smoke suppression design.     

Influence of Microstructure and Surface Activation of Dual‐Phase Membrane Ce0.8Gd0.2O2−δ–FeCo2O4 on Oxygen Permeation

Dual‐phase oxygen transport membranes are fast‐growing research interest for application in oxyfuel combustion process. One such potential candidate is CGO‐FCO (60 wt% Ce_0.8Gd_0.2O_2?δ–40 wt% FeCo₂O₄) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO‐FCO membranes of 1 mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one‐pot method (modified Pechini process) at 1200°C for 10 h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. It was also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) porous layer coating over the composite. The oxygen permeation flux of the CGO‐FCO screen printed with a porous layer of 10 μm thick LSCF is 0.11 mL/cm² per minute at 850°C with argon as sweep and air as feed gas at the rates of 50 and 250 mL/min.